Understanding the Pipeline

The pipeline is the heart of PowerShell. It lets you chain commands together so the output of one feeds directly into the input of the next — as objects, not text.

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The Pipe Operator |

Command-One | Command-Two | Command-Three

Each command receives the objects produced by the previous command.

A simple example

Get-Process | Sort-Object CPU -Descending | Select-Object -First 5

Step by step:

1. Get-Process — returns a collection of Process objects
2. | Sort-Object CPU -Descending — sorts them by the CPU property, highest first
3. | Select-Object -First 5 — takes only the first 5 results

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How Pipeline Input Works

When you pipe into a cmdlet, PowerShell binds the incoming objects to parameters marked with ValueFromPipeline or ValueFromPipelineByPropertyName.

By value

"notepad","calc" | Start-Process

Start-Process accepts strings by value, so each string becomes the -FilePath.

By property name

Import-Csv .\servers.csv | Test-Connection -Count 1

If the CSV has a column named ComputerName, PowerShell automatically binds it to Test-Connection's -ComputerName parameter.

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The $_ / $PSItem Variable

Inside ForEach-Object and Where-Object script blocks, $_ (or its alias $PSItem) refers to the current pipeline object:

Get-ChildItem *.log | ForEach-Object {
    "Processing: $($_.Name) ($($_.Length) bytes)"
}

Get-Service | Where-Object { $_.Status -eq 'Running' }
# Shorthand syntax (PS 3+):
Get-Service | Where-Object Status -eq Running

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Essential Pipeline Cmdlets

Cmdlet	Purpose
Where-Object / ?	Filter objects by condition
ForEach-Object / %	Run code for each object
Select-Object	Pick properties or first/last N objects
Sort-Object	Order objects by property
Group-Object	Group objects sharing a property value
Measure-Object	Count, sum, average, min, max
Tee-Object	Split output to file/variable AND the pipeline
Out-File	Send formatted output to a file
Export-Csv	Serialize to CSV
ConvertTo-Json	Serialize to JSON

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Building Complex Pipelines

Example: Disk space report

Get-PSDrive -PSProvider FileSystem |
    Where-Object { $_.Used -gt 0 } |
    Select-Object Name,
        @{Name='Used GB';  Expression={ [math]::Round($_.Used  / 1GB, 2) }},
        @{Name='Free GB';  Expression={ [math]::Round($_.Free  / 1GB, 2) }},
        @{Name='Total GB'; Expression={ [math]::Round(($_.Used + $_.Free) / 1GB, 2) }} |
    Sort-Object 'Total GB' -Descending |
    Format-Table -AutoSize

Example: Top memory consumers

Get-Process |
    Sort-Object WorkingSet64 -Descending |
    Select-Object -First 10 Name, Id,
        @{Name='MB'; Expression={ [math]::Round($_.WorkingSet64 / 1MB, 1) }} |
    Format-Table -AutoSize

Example: Export stopped services

Get-Service |
    Where-Object Status -eq Stopped |
    Select-Object Name, DisplayName, StartType |
    Export-Csv .\stopped-services.csv -NoTypeInformation

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Calculated Properties

Use a hashtable with Name and Expression keys inside Select-Object or Sort-Object to create custom computed properties:

Get-ChildItem C:\Windows -File |
    Select-Object Name,
        @{Name='SizeKB'; Expression={ [math]::Round($_.Length / 1KB, 1) }},
        LastWriteTime |
    Sort-Object SizeKB -Descending |
    Select-Object -First 20

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Saving Pipeline Results

To a variable

$procs = Get-Process | Where-Object CPU -gt 5
$procs.Count   # how many?
$procs | Format-Table Name, CPU

To a file

Get-Process | Out-File .\processes.txt
Get-Process | Export-Csv .\processes.csv -NoTypeInformation
Get-Process | ConvertTo-Json | Out-File .\processes.json

Both at once with Tee-Object

Get-Service |
    Tee-Object -FilePath .\services.txt |
    Where-Object Status -eq Running |
    Measure-Object

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Pipeline Tips

Use -WhatIf before destructive operations

Get-ChildItem *.tmp | Remove-Item -WhatIf

-WhatIf shows what would happen without actually doing it.

Use Out-GridView for interactive exploration

Get-Process | Out-GridView

This opens an interactive filterable table — great for ad-hoc analysis.

Format-* ends the pipeline

Format-Table, Format-List, and Format-Wide convert objects to formatting directives. After them, you can't pipe to Export-Csv or Select-Object. Always format last.

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Next Step

Variables & Data Types